Abstract: :
Purpose: To develop a software technique to define and extractthe functional optical zone from topographic data after refractivesurgery.Methods: A retrospective study of 52 eyes 29 patients who receivedKeratron Scout corneal topography (Optikon, Rome, Italy) andZywave wavefront measurements (Bausch&Lomb, Rochester, NewYork) three months post–LASIK using one of three laserswas conducted. The acquired data were processed using The OhioState University Corneal Topography Tool (OSUCTT) to generaterefractive power maps. Next, a region growing algorithm wasdeveloped and applied to determine the size and location ofthe functional optical zone (FOZ). First, the flattest areabounded by a 2 mm diameter circle was located and used as aseed point for the region growing. Then, an algorithm basedon a bayesian classifier was used to define the FOZ. The centroidof this region and its distance from the center of the map (decentration)was calculated. Also, the average refractive power inside (ref_oz),outside (ref_out) of the FOZ, and the difference (ref_diff =ref_oz – ref_out) were calculated. Linear regression analysiswas performed to analyze the relationship of decentration tocoma and ref_diff to spherical aberration. Coma was calculatedfrom the wavefront measurements as being the square root ofthe sum of the squares of Z3–1 and Z31. The term Z40 wasused to represent spherical aberration.Results: Decentration of the FOZ significantly correlated withcoma (p < 0.01, r2=0.18). Ref_diff significantly correlatedwith spherical aberration (p < 0.01, r2=0.33). Average FOZwas 28.5 mm2 with a major axis of 6.35 mm and a minor axis of5.71mm.Conclusions: Size, decentration, and relative magnitude of FOZcalculated from refractive power maps based on topographic measurementsprovides important information about optical aberrations inducedby refractive surgery.